[jogibear9988]

Nested initialization currently mutates readonly struct fields. Load a readonly value field into a local copy before applying its nested bindings, preserving the stored field as System compilation does.

Readonly instance struct fields, with writable-field controls and System compilation as the behavior oracle. Regressions require strict FEC compilation. Native suites cover Standard and LightExpression.

Validation

Local validation uses master baseline 094d067 plus this change. Release solution build: zero warnings/errors.

Runtime	TestX groups	Legacy tests
.NET 10	489 passed	1,756 passed
.NET 8	489 passed	1,755 passed
.NET Framework 4.7.2	468 passed	1,639 passed

Measurements

Master baseline 094d0678 → this change, Release, Windows ARM64, .NET 8.0.28 / 10.0.9. Compilation uses prebuilt trees; runtime measures invocation through the same wrapper. Two fresh process rounds with reversed load order, seven samples per round, about 30 ms calibration, one logical CPU and tiered compilation disabled. Tables use the median of the two per-round medians.

The controls below produce correct results on master for their measured inputs. Newly supported trees have no valid before/after execution comparison because master rejects them, produces incorrect results or emits invalid IL. Timings include machine and JIT/code-placement variation; the unchanged add control helps show that variation. These observations do not prove universal performance neutrality.

net8.0 results (master → change)

API / workload	Compile ns	Invoke ns	Compile B/op	Invoke B/op
Standard / add_control	1556.8 → 1603.4	2.5 → 2.6	872.0 → 872.0	0.0 → 0.0
Standard / member_init	2161.7 → 2648.0	4.7 → 5.3	1064.1 → 1064.1	24.0 → 24.0
Standard / nested_writable_field	1632.9 → 1855.3	6.8 → 4.6	1064.1 → 1064.1	24.0 → 24.0
Light / add_control	1749.8 → 1424.4	2.4 → 2.4	872.0 → 872.0	0.0 → 0.0
Light / member_init	2434.2 → 2350.6	15.3 → 7.3	1064.0 → 1064.0	24.0 → 24.0
Light / nested_writable_field	2410.9 → 2592.6	15.4 → 15.0	1064.0 → 1064.0	24.0 → 24.0

net10.0 results (master → change)

API / workload	Compile ns	Invoke ns	Compile B/op	Invoke B/op
Standard / add_control	3608.4 → 3595.1	2.2 → 2.2	872.0 → 872.0	0.0 → 0.0
Standard / member_init	4327.8 → 4505.9	4.2 → 4.1	1064.2 → 1064.2	24.0 → 24.0
Standard / nested_writable_field	3596.7 → 3538.5	4.1 → 4.1	1064.2 → 1064.2	24.0 → 24.0
Light / add_control	2699.3 → 2773.6	2.2 → 2.1	872.0 → 872.0	0.0 → 0.0
Light / member_init	3106.0 → 3286.2	3.9 → 3.9	1064.0 → 1064.0	24.0 → 24.0
Light / nested_writable_field	2756.8 → 2778.0	4.0 → 4.0	1064.0 → 1064.0	24.0 → 24.0

Emitted IL differs in these measured workloads: none.
No measured allocation increase above 0.1 B/op in these controls.